The 1960 street map of Lagos, Nigeria, shows a small western-style coastal city surrounded by a few semi-rural African villages. Paved roads quickly turn to dirt, and fields to forest. There are few buildings over six floors high and not many cars.

No one foresaw what happened next. In just two generations Lagos grew 100-fold, from under 200,000 people to nearly 20 million. Today one of the world’s 10 largest cities, it sprawls across nearly 1,000 sq km. Vastly wealthy in parts, it is largely chaotic and impoverished. Most residents live in informal settlements, or slums. The great majority are not connected to piped water or a sanitation system. The city’s streets are choked with traffic, its air is full of fumes, and its main dump covers 40 hectares and receives 10,000 metric tons of waste a day.

Q&A

What is the Overstretched Cities series?

Overstretched Cities is an in-depth look at how urbanisation has seen cities all over the world mushroom in size, putting new strain on infrastructure and resources – but in some cases offering hope for a more sustainable relationship with the natural world.

Over the course of the week, Guardian Cities correspondents will look beyond the numbers to tell the stories of people affected by the 21st century's population and consumption boom. From sprawling cities in the developed world that are consuming more than their fair share of energy and water, to less wealthy cities unequipped to handle the rapid increase in geographical and population size, we will interrogate this global phenomenon by talking to the people who are trying to mitigate its worst effects – and shine a light on the upside of our population boom by exploring the social and environmental advantages of urban density.

Was this helpful?

Thank you for your feedback.

But new research suggests that the changes Lagos has seen in the last 60 years may be nothing to what might take place in the next 60. If Nigeria’s population continues to grow and people move to cities at the same rate as now, Lagos could become the world’s largest metropolis, home to 85 or 100 million people. By 2100, it is projected to be home to more people than California or Britain today, and to stretch hundreds of miles – with enormous environmental effects.

Makoko, a slum located in a lagoon in Lagos, Nigeria. Photograph: Frédéric Soltan/Corbis via Getty Images

Hundreds of far smaller cities across Asia and Africa could also grow exponentially, say the Canadian demographers Daniel Hoornweg and Kevin Pope at the University of Ontario Institute of Technology. They suggest that Niamey, the barely known capital of Niger – a west African country with the highest birth rate in the world – could explode from a city of fewer than one million people today to be the world’s eighth-largest city, with 46 million people, in 2100. Sleepy Blantyre in southern Malawi could mushroom to the size of New York City today.

Is the way we think about overpopulation racist?

Read more

Under the researchers’ extreme scenario – where countries are unable to control fertility rates and urbanisation continues apace – within 35 years more than 100 cities will have populations larger than 5.5 million people. By 2100, say the authors, the world’s population centers will have shifted to Asia and Africa, with only 14 of the 101 largest cities in Europe or the Americas.

What happens to those cities over the next 30 years will determine the global environment and the quality of life of the world’s projected 11 billion people. It’s impossible to know how exactly how cities will grow, of course. But the stark fact, according to the United Nations, is that much of humanity is young, fertile and increasingly urban. The median age of Nigeria is just 18, and under 20 across all Africa’s 54 countries; the fertility rate of the continent’s 500 million women is 4.4 births. Elsewhere, half of India’s population is under age 25, and Latin America’s average age is as high as 29.

Lagos, set to become the largest metropolis the world has ever known. Photograph: Pius Utomi Ekpei/AFP/Getty Images

Latest UN projections expect the world’s population to grow by 2.9 billion – equal to another China and India – in the next 33 years, and possibly by a further three billion by the end of the century. By then, says the UN, humanity is expected to have developed into an almost exclusively urban species with 80-90% of people living in cities.

Cities in numbers: how patterns of urban growth change the world

Read more

Whether those cities develop into sprawling, chaotic slums – with unbreathable air, uncontrolled emissions and impoverished populations starved of food and water – or become truly sustainable depends on how they respond. Many economists argue that population growth is needed to create wealth, and that urbanisation significantly reduces humanity’s environmental impact. Other observers fear cities are becoming ungovernable – too unwieldy to adapt to rising temperatures and sea levels, and prone to pollution, water shortages and ill health.

Many cities are already investing in clean transport and water, sewage, renewable energy, planning, wellbeing and good housing for all. Others face what seem like insurmountable problems.

Bangalore, India

“This city was renowned for its trees, lakes and pleasant air only 25 years ago,” says TV Ramachandra, head of the Energy and Wetlands Research Group at the Indian Institute of Science. “Now it is a dead city, which has sacrificed its environment for some of the fastest economic growth seen anywhere in the world.”

India, which is widely expected to be the most populous country in the world with more than 1.5 billion people by 2050, has seen its urban population double in 30 years, to nearly 600 million. Its megacities, like Mumbai and Delhi, are not expected to grow much more; instead, smaller cities are rapidly expanding.

Bangalore is the worst city in the world for unchecked urbanisation

TV Ramachandra, Indian Institute of Science

Ramachandra and his colleague Bharath H Aithal have documented the environmental effects of breakneck urban growth in Bangalore. They say temperature in the city has increased by 2-2.5C over the past three decades, while the water table has declined in places from 28 metres down to 300 metres deep; there has been an 88% loss of vegetation and a 79% loss in wetlands, and frequent flooding even during normal rainfall.

Ramachandra fears that what has happened to Bangalore will happen to all Indian cities. “Air pollution is at dangerous levels, the water is polluted, there is nowhere for the waste to go, and the lakes have been killed,” he says.

The “frenzy of unplanned urbanisation” is threatening nature as never before, says Prerna Bindra, author of The Vanishing, a new analysis of how urbanisation and economic growth have affected India’s rich wildlife. “Wetlands, lakes, green spaces are giving way to glass and concrete. The retreat of natural habitats has meant the rapid decline of urban wildlife – even the once ubiquitous – house sparrows, or the bullfrogs and common toads that serenaded the monsoons, or jackals [which were] once not a very uncommon sight on urban fringes.”

The solution may be in the hands of the many strong indigenous and middle class groups that have set up in the last 20 years to demand less destructive development and attempt to reduce the use of polluting fossil fuels, enforce conservation laws and educate the authorities. But there is a long way to go.

“The situation is very worrying. People are moving out. Illnesses are increasing. At this rate every house will need a dialysis machine,” Ramachandra says. “Bangalore cannot continue like this. It is becoming an unliveable city. This is the worst city in the world for unchecked urbanisation.”

Kinshasa, DRC

A boy crosses a river whose banks are littered with rubbish in the Kinshasa neighbourhood of Ngaliema. Photograph: John Wessels/AFP/Getty Images

Pierre Sass moved to Kinshasa in the 1990s. Like thousands of young men, he came looking for work, rented an overcrowded room from a “brother” on the edge of the city and, four years later, bought his own plot of land. Today, the edge of the city is 3 miles (five kilometres) away and he has built his own two-room house for himself, his wife and three children. He has electricity but no water or drainage.

Kinshasa had just 20,000 people in 1920. By 1940 it was home to about 450,000 people. Today it has possibly 12 million and is predicted to be Africa’s second largest city with 75 million people inside 50 years. By western standards it is a dysfunctional, sprawling megalopolis, ringed by vast shantytowns of informal settlements, their infrastructure nonexistent or collapsing.

“When you go to there today you see disarray and congestion,” says Somik Lall, the World Bank’s lead economist for Africa. “Yes, it will be one of the biggest cities in Africa by 2050, but I do not think it is the model for future Africa, nor do I think it will have a population of 70 million.” He argues that Kinshasa’s current condition is not necessarily indicative of its future status. “There’s no way to say what cities will look like in 2100. Seoul in 1980 could never have predicted how it is today. It was grimy, dirty, industrial city. Africa has a young labour force. Places like Kinshasa are some of the most dynamic places in the world.”

Where the urban sprawl of Kinshasa meets the surrounding countryside. Photograph: FredR/Flickr

He worries, however, that economic growth will not keep up with population growth, as it did in industrialising Asia, Europe and the US. “What seems to be happening in Africa is that it is triggering only small-scale informal trading [as opposed to global commerce]. People coming to cities like Kinshasa are not adding economic benefit. Not enough investment is being made in the infrastructure of African cities.”

By 2100, about 40% of all humans and nearly half of all children in the world will be African – one of the fastest and most radical demographic changes in history. It is bound to be a messy transition, Lall says. “But I am not worried about the grime and dirt so much. That comes later. We mix up wanting a city to be productive and be pretty; I want to make sure people get a good job.”

Guiyang, China

Tan Guo is 24 years old and bewildered. After years living away from Guiyang, she returned last year from Germany to find the formerly small provincial capital had become China’s fastest-growing city, and completely unrecognisable.

The fields her grandparents worked, once over eight kilometres from the small city centre, are now covered with closely packed high-rises. Her family members live on the eighth floor, looking over shopping malls and ring roads. Former slums have been razed, farmland converted, rivers diverted and forests felled.

The city stretches so far. It is like a new world where I know no​​ one and nothing

Tan Guo, returning resident

“Even the hills look different. I was really shocked. The city stretches so far. It is like a new world where I know no one and nothing,” she says.

The scale and speed of China’s shift to cities is shocking – possibly the fastest and largest migration of a human population in history. In just 30 years, nearly 500 million people have moved from rural areas into China’s 622 main cities, and a predominantly rural country has become nearly 60% urban. By 2025, over one billion Chinese – two in three people – will live in cities.

Guiyang is a model of central urban planning from the perspective of people. It has few slums and little sprawl, and its growth has been ordered. But urbanisation has been an ecological disaster. In the early days, pollution turned the Nanming river black and stinking. Air pollution was allowed to continue unchecked, while carbon dioxide emissions rocketed from coal-fired industry, forests were cleared and soil was contaminated on a massive scale. And China’s breakneck urbanisation extends beyond its borders, devastating vast areas of Africa and Latin America, where it turned for the raw materials for its industrial revolution.

Tourists pose for a photo in Quanhu lake park in Guiyang. Photograph: Alamy

Today, Tan Guo’s Guiyang is still growing at breakneck speed, but authorities are trying to rectify some of the mistakes – though it would take generations to do so. The city has spent $150m cleaning up the Nanming river and has restricted new car sales and set quotas on electric car numbers to reduce air pollution.

“Rapid urbanisation was encouraged. It was the way China grew its economy,” says Gordon McGranahan of the Institute of Development Studies in the UK, who specialises in global urbanisation. “China used cities to generate growth and land to generate investment. It had to bring people to the cities; it experimented with converting land to urban areas. Its cities were critical to its growth. No one paid much attention to the environment until it hit them in the face.”

Mexico City

Priscilla Connolly has lived in Mexico City since the 1970s. In that time, she has seen it triple in size, into one of the world’s five largest cities.

“No one expected Mexico City to grow so much,” says Connolly, a professor of urban sociology at the Universidad Autónoma Metropolitana. “But for years it was thought that the more people who lived there the better. There was a pro-population policy. Now the city has stopped growing and medium-sized cities are growing fastest.”

The city is still vastly overcrowded, massively polluted and predominantly poor, with little space to build the 50,000 new houses a year that it needs. But it shows that rapid change can be controlled, and that urbanisation has its benefits.

“There has been progress in so many ways,” Connolly says. “Most people can read and are housed. They don’t expect to die at five. All in all it’s been a successful transition, though fraught with future environmental risk.”

Fast-growing cities in Africa and Asia could learn from Mexico City’s mistakes, Connolly says. “Planning and thinking was geared to the idea that cars could circulate. Only 30% of Mexico City has a car, but the city was designed for the car. The 19th-century sanitary revolution has to be rethought. The environmental impacts of urbanisation are much worse outside cities.”

The shantytowns and informal settlements that ringed Mexico City in the 1970s are now being upgraded. However, environmental issues are still not high on the agenda, and the city has a semi-permanent water crisis.

“Water will be the crunch issue here. Will there be enough water? Probably not,” Connolly says. “People will have to reduce consumption. It will need more aggressive policies. Cities must think about the whole water cycle. In 50 years’ time, wastewater will be like gold.”

El Alto, Bolivia

Poor El Alto is the satellite city of rich La Paz. In 1952, it was little more than a village, overlooking the capital of Bolivia from the high altiplano. In 1960, it had fewer than 30,000 people. Now it has exploded into a metropolis with one million or more people living in the rarified air 4,100 metres (13,000 feet) above sea level.

Hundreds of thousands of people, including Mario Mamani, have flooded in over the last decade. Mamani and his family came from Pinaya, a village high in the Andes below the snowcapped Illimani mountain, nearly 10 years ago. He wanted work, and frequent droughts, erratic rainfall, heat waves, unseasonal frosts and floods had made conditions too hard to grow crops.

El Alto is a city in progress, he says, starting to prosper as industry and its economy develop. It is also a model of how to build from scratch in a bleak environment with few resources. Latin America is now the world’s most urbanised region, with 90% of people living in towns and cities. In 50 years time, if it can get enough water, El Alto is likely to outgrow La Paz itself.

“Mexico, Brazil, Argentina and Uruguay led the urbanisation of Latin America,” says urban researcher Fernando Aragón-Durand, lead author for the Intergovernmental Panel on Climate Change’s Special Report on meeting the UN’s 1.5C temperature target. “But we are still trying to understand the implications of its effect on the environment.”

Copenhagen

European cities are some of the world’s richest, and most are expected to barely grow or even shrink over the next 50 years. Declining birth rates, ageing populations and good infrastructure allow many to now focus on the environment.

But even if they aren’t growing at nearly the rate of other cities, they are some of the world’s biggest consumers of energy and resources and emissions. This, says Copenhagen lord mayor Frank Jensen, imposes on them an ethical duty to change.

“Today our harbour is so clean we can swim in the water, and 62% of Copenhageners ride their bike to work or school every day. I want Copenhagen to maintain and further develop this position as a green and liveable city,” he says.

The city has set the ambitious goal of becoming the world’s first carbon neutral capital by 2025. As well as tackling energy production and consumption, “this would mean building even more bicycle lanes and new bridges to make even more Copenhageners choose the bike over the car,” he says.

Jensen thinks cities are more adaptable and quicker to act than higher levels of government. He envisages them exchanging ideas and initiatives, through clusters like C40 Cities, an international forum for mayors to meet and develop new ideas to solve environmental problems.

In Europe, where one in four people is aged 60 or over – a figure expected to rise to 36% by the turn of the century – the problem isn’t too many people, but too much strain. “By sharing our best ideas and solutions, we can go from great solutions locally to brilliant advances globally,” says Jensen. “Cities therefore play a key role in ensuring a more sustainable future.”

Kigali, Rwanda

Believe the graphics and the spreadsheets, and Kigali, the capital of Rwanda, will by 2040 be a cross between Singapore and Dallas. The city’s master plan, drawn up by Asian and Australian architects in 2013, suggests its many slums will be cleared, new financial and university districts will be built and there will be a new airport, ring roads, gleaming high-rise towers, urban farms, pleasant parks and good housing for hundreds of thousands of people.

Four years on, the plan is still a fantasy. The international investors have failed largely to come to the world’s 25th poorest country. The roads and public transport are not built. There are persistent water shortages, power cuts and gridlock. The city-centre slums are being cleared to make way for the developers, but otherwise Kigali is still just a desperately poor capital city.

African cities cannot become sustainable by building roads. They must embrace public transport

Vanessa Watson, Cape Town University

Across Africa, hundreds of smaller cities are seeing population growth of 3.5% a year or more, doubling in size every 21 years. Large cities such as Accra, Lagos, Kinshasa, Entebbe and Nairobi, meanwhile, all have plans to attract their rich and middle classes to shiny new satellite cities and districts.

But in an area of sub-Saharan Africa where the population could well double in 50 years, Kigali is one of the few cities determined to limit growth.

“Cities in Africa have been deprived of state investment. The wealth of many countries has been directed abroad,” says Vanessa Watson, professor of city planning at Cape Town University. “Rwanda and Ethiopia alone are controlling urban populations, by destroying slums and moving people to smaller towns.”

Watson fears that skewing state spending towards ambitious new developments and satellite cities for the middle class is neglecting poorer inner-city areas, where people are being evicted from well-located spots. Her recommendation: “African cities must learn that they cannot become sustainable by building roads. They must embrace public transport and not cars. They must find their own model and not try to reproduce European cities, which were built on vast capital and centuries of slow growth. If not, there is a bleak future of growing inequality and massive environmental damage.”